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Exercise: It's an hour a day, people. Seriously.

That 30 minutes of daily exercise you think you’re supposed to do to keep weight off? You need to step it up, people. As much as twice that amount may be needed to lose weight and keep it off.

A recent study found that overweight and obese women needed to exercise about an hour a day, five days a week to sustain weight loss. The findings bolster what some health experts — and those who have lost weight and kept it off — have been saying for years: copious amounts of exercise and adherence to a strict diet are necessary to take off the pounds and keep them at bay.

The women who exercised more and stuck to their diets kept off a 10% weight loss over two years, compared with others who maintained only 5%. The report, which appeared in the July 28 issue of Archives of Internal Medicine, studied 201 women during an intervention that spanned from 1999 to 2003. All the women were asked to consume 1,200 to 1,500 calories a day, and they were assigned to one of four groups: one that burned 1,000 calories a week, one that burned 2,000 calories a week, one that exercised moderately and one that exercised vigorously. Participants also attended group meetings where they learned how to change their diet and activity and received follow-up calls via telephone.

Six months later, all four groups had lost an average of 8% to 10% of their body weight. But it didn’t last. After two years, the average weight of all participants was only 5% lower than their initial weight, and there was no difference among the groups.

But some did better than average. About a quarter of the women who managed to sustain a 10% weight loss exercised more, adhered to better eating habits and engaged more often by phone with the intervention team. For them, exercise amounted to an average of expending 1,835 calories a week, or 275 minutes per week.

"This clarifies the amount of physical activity that should be targeted for achieving and sustaining this magnitude of weight loss, but also demonstrates the difficulty of sustaining this level of physical activity," wrote the authors (headed up by John Jakicic, director of the Physical Activity and Weight Management Research Center at the University of Pittsburgh), who also recommended further research to discover how to continue to motivate people to exercise.